An official website of the United States government
This research investigates the dynamic response of a novel polyurea foam with different densities by separately submitting samples to single and multiple impacts at different energies ranging from 1.77 to 7.09 J. The impact and transmitted force‐time histories are acquired during the impact events. Deformation of the samples is also recorded using high‐speed photography and analyzed using digital image correlation (DIC) to characterize density‐dependent strain rate and Poisson's ratio. The analyses of the force‐time histories highlight the interrelationship between the incoming impact energy and force characteristics, including amplitude and durations. The experimental results reveal that polyurea foams can absorb nearly 50% of the incoming impact energy irrespective of their density. The dynamic impact efficacy of the foam persists even after sequential impact events are imparted on the same samples, with only a 20% drop in the load‐bearing capacity after seven consecutive impacts. Furthermore, as verified via electron microscopy observations, the higher‐density foam does not exhibit any permanent damage. This high‐density polyurea foam shows reversible auxetic transition at all impact energies considered herein. The outcomes of this research indicate the suitability of polyurea foams for cushioning and impact mitigation applications, especially in repeated biomechanical impact scenarios.
more »
« less
In-plane density gradation of shoe midsoles for optimal energy absorption performance
Midsoles are important components in footwear as they provide shock absorption and stability, thereby improving comfort and effectively preventing certain foot injuries. A strategically engineered midsole designed to mitigate plantar pressure can enhance athletic performance and comfort levels. Despite the importance of midsole design, the potential of using in-plane density gradation (deliberate variation of material density across the horizontal plane) in midsoles has been rarely explored. The present work investigated the effectiveness of in-plane density gradation in shoe midsoles using novel polyurea foams as the material candidate. Different polyurea foam densities, ranging from 95 to 350 kg/m2were examined and tested to construct density-dependent correlative mathematical relations required for optimizing the midsole design for enhanced cushioning and reduced weight. This study combined mechanical testing and plantar pressure measurements to validate the efficacy of density-graded midsoles. The methodology introduced here is relevant to realistic walking conditions, ensured by biomechanical tests supplemented by digital image correlation analyses. An optimization framework was then created to allocate foam densities at certain plantar zones based on the required cushioning performance constrained by the local pressure. The optimization algorithm was specifically tailored to accommodate varying local pressures experienced by different areas of the foot. The optimization strategy in this study aimed at reducing the overall weight of the midsole while ensuring there were no compromises in cushioning efficacy or distribution of plantar pressure. The approach presented herein has the potential to be applied to a wide range of gait speeds and user-specific plantar pressure patterns.
more »
« less
- Award ID(s):
- 2035660
- PAR ID:
- 10542697
- Publisher / Repository:
- SAGE Publications
- Date Published:
- Journal Name:
- Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology
- ISSN:
- 1754-3371
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Density‐graded elastomeric foams are emerging as effective protective structures to guard humans against mechanical loading. This research investigates the deformation of ungraded and graded foams under quasistatic and impact scenarios using digital image correlation (DIC). The graded samples are assembled using two interfacing strategies (seamless and adhered), leveraging the adhesiveness of the foam slurry and bulk polyurea, respectively. Deformation mechanisms, including the effect of the interface type on strain transduction and localization in density‐graded structures, are imperative for improving the impact efficacy of protective paddings. Cuboid foam plugs are subjected to quasistatic and impact loading while recording the corresponding deformation for DIC analysis. The DIC results are separated into three case studies based on the number of layers (1, 2, and 3). The interface effect on the overall mechanical performance of polyurea foam is revealed from the bilayer, monodensity samples, showing drastic differences between the deformations within each layer. Seamless interface samples exhibit greater compliance than their adhered counterparts in the bilayer density‐graded configurations. Trilayer‐graded foams broaden strain–time history, extend the impact duration, and reduce strains. This research substantiates the importance of interfacing and gradation strategies on the mechanical response of elastomeric foams as a function of strain rate.more » « less
-
Though the rabbit is a common animal model in musculoskeletal research, there are very limited data reported on healthy rabbit biomechanics. Our objective was to quantify the normative hindlimb biomechanics (kinematics and kinetics) of six New Zealand White rabbits (three male, three female) during the stance phase of gait. We measured biomechanics by synchronously recording sagittal plane motion and ground contact pressure using a video camera and pressure-sensitive mat, respectively. Both foot angle ( i.e ., angle between foot and ground) and ankle angle curves were unimodal. The maximum ankle dorsiflexion angle was 66.4 ± 13.4° (mean ± standard deviation across rabbits) and occurred at 38% stance, while the maximum ankle plantarflexion angle was 137.2 ± 4.8° at toe-off (neutral ankle angle = 90 degrees). Minimum and maximum foot angles were 17.2 ± 6.3° at 10% stance and 123.3 ± 3.6° at toe-off, respectively. The maximum peak plantar pressure and plantar contact area were 21.7 ± 4.6% BW/cm 2 and 7.4 ± 0.8 cm 2 respectively. The maximum net vertical ground reaction force and vertical impulse, averaged across rabbits, were 44.0 ± 10.6% BW and 10.9 ± 3.7% BW∙s, respectively. Stance duration (0.40 ± 0.15 s) was statistically significantly correlated ( p < 0.05) with vertical impulse (Spearman’s ρ = 0.76), minimum foot angle ( ρ = −0.58), plantar contact length ( ρ = 0.52), maximum foot angle ( ρ = 0.41), and minimum foot angle ( ρ = −0.30). Our study confirmed that rabbits exhibit a digitigrade gait pattern during locomotion. Future studies can reference our data to quantify the extent to which clinical interventions affect rabbit biomechanics.more » « less
-
Polymeric foams, embedded with nano-scale conductive particles, have previously been shown to display quasi-piezoelectric (QPE) properties; i.e., they produce a voltage in response to rapid deformation. This behavior has been utilized to sense impact and vibration in foam components, such as in sports padding and vibration-isolating pads. However, a detailed characterization of the sensing behavior has not been undertaken. Furthermore, the potential for sensing quasi-static deformation in the same material has not been explored. This paper provides new insights into these self-sensing foams by characterizing voltage response vs frequency of deformation. The correlation between temperature and voltage response is also quantified. Furthermore, a new sensing functionality is observed, in the form of a piezoresistive response to quasi-static deformation. The piezoresistive characteristics are quantified for both in-plane and through-thickness resistance configurations. The new functionality greatly enhances the potential applications for the foam, for example, as insoles that can characterize ground reaction force and pressure during dynamic and/or quasi-static circumstances, or as seat cushioning that can sense pressure and impact.more » « less
-
null (Ed.)This paper reports the unique microstructure of polyurea foams that combines the advantages of open and closed cell polymeric foams, which were synthesized through a self-foaming process. The latter was the result of aggressive mechanical mixing of diamine curative, isocyanate, and deionized water at ambient conditions, which can be adjusted on-demand to produce variable density polyurea foam. The spherical, semi-closed microcellular structure has large perforations on the cell surface resulting from the concurrent expansion of neighboring cells and small holes at the bottom surface of the cells. This resulted in a partially perforated microcellular structure of polyurea foam. As a byproduct of the manufacturing process, polyurea microspheres nucleate and deposit on the inner cell walls of the foam, acting as a reinforcement. Since cell walls and the microspheres are made of polyurea, the resulting reinforcement effect overcomes the fundamental interfacial issue of different adjacent materials. The partially perforated, self-reinforced polyurea foam is compared to the performance of traditional counterparts in biomechanical impact scenarios. An analytical model was developed to explicate the stiffening effect associated with the reinforcing microspheres. The model results indicate that the reinforced microcell exhibited, on average, ~30% higher stiffness than its barren counterpart.more » « less
